Many managers mistakenly believe that employees must sign written
warnings in order for those warnings to be valid. That’s not really the case. The purpose of progressive discipline is to communicate problematic performance issues in an effort to raise employees’ awareness so that performance improves.

When an employee is given a written warning (or letter of clari-
fication, for that matter), several things may occur:
The employee may sign the document on the spot.
The individual may ask for several days’ time to review the document before signing it in order to write a rebuttal.
The worker may simply refuse to sign the document without further discussion.

All of these options are acceptable.What is important, however, is the documentation of the events. Even with goodwill efforts and suggestions that you put forth, employees may still simply refuse to
sign.When that’s the case, it’s time to call in a witness. The witness’s
role is simply to verify that the document was given to the employee.
This way, if an employee later denies having been given the warning, the witness can refute that claim.

Tell Me More
Let’s briefly look at the three circumstances. When employees sign
a written warning, they formally recognize that they received it.
Therefore, they can’t deny later in court that they were given ample
notification of the problem and a chance to provide their own side
of the story via a rebuttal.

Taken From : The Hiring and Firing Quention and Answer Book